Stents and stent delivery devices are employed in a number of medical procedures and as such their structure and function are well known. Stents are used in a wide array of bodily vessels including coronary arteries, renal arteries, peripheral arteries including iliac arteries, arteries of the neck and cerebral arteries as well as in other body structures, including but not limited to arteries, veins, biliary ducts, urethras, fallopian tubes, bronchial tubes, the trachea, the esophagus and the prostate.
Stents are typically cylindrical, radially expandable prostheses introduced via a catheter assembly into a lumen of a body vessel in a configuration having a generally reduced diameter, i.e. in a crimped or unexpanded state, and are then expanded to the diameter of the vessel. In their expanded state, stents support or reinforce sections of vessel walls, for example a blood vessel, which have collapsed, are partially occluded, blocked, weakened, or dilated, and maintain them in an open unobstructed state. To be effective, the stent should be relatively flexible along its length so as to facilitate delivery through torturous body lumens, and yet stiff and stable enough when radially expanded to maintain the blood vessel or artery open. Such stents may include a plurality of axial bends or crowns adjoined together by a plurality of struts so as to form a plurality of U-shaped members coupled together to form a serpentine pattern.
There are two types of stents that are presently utilized: permanent stents and bioabsorbable stents. A permanent stent is designed to be maintained in a body lumen for an indeterminate amount of time. Permanent stents are typically designed to provide long-term support for damaged or traumatized wall tissues of the lumen. There are numerous conventional applications for permanent stents including cardiovascular, peripheral, urological, gastrointestinal, and gynecological applications.
Bioabsorbable stents may advantageously be eliminated from body lumens after a predetermined, clinically appropriate period of time, for example, after the traumatized tissues of the lumen have healed and a stent is no longer needed to maintain the integrity of the lumen. The conventional bioabsorbable materials from which such stents are made are selected to resorb or degrade over time, thereby eliminating the need for subsequent surgical procedures to remove the stent from the body lumen if problems arise.
One technique that is employed to manufacture stents is laser cutting. Laser cutting of stents has been described in a number of publications including U.S. Pat. No. 5,780,807 to Saunders, U.S. Pat. No. 5,922,005 to Richter and U.S. Pat. No. 5,906,759 to Richter. Laser cutting usually involves the use of a pulsed laser beam and a stent preform such as a tubular preform that is positioned under the laser beam and moved in a precise manner to cut a desired pattern into the preform using a servo motion controlled machine tool. Laser cutting is sometimes performed in an oxygen atmosphere to assist in the process. An example of a conventional laser for cutting a stent is a highly focused pulsed Nd:YAG laser which has a pulse duration in the range of approximately 0.1 to 20 milliseconds. The laser produces a relatively large melt zone and heat affected zone (HAZ) on the metal.
To reduce the size of the heat affected zone, cutting and processing systems have been developed that incorporate a water column and laser. For example, SYNOVA Inc., of Lausanne, Switzerland, has developed a laser-microjet that uses a laser beam that is contained within a water jet arranged as a parallel beam, similar in principle to an optical fiber. The SYNOVA laser-microjet relies on a low pressure water column to contain the laser, to reduce force applied to the work piece, to act as a cooling mechanism and to remove cutting debris. In U.S. Pat. No. 6,696,666, a laser microjet is employed to manufacture a stent. A second water jet is directed through the tubular preform to deflect the laser beam, thereby preventing damage to the interior wall of the tubular preform.